Mass wasting and sediment storage in a small montane watershed: An extreme case of anthropogenic disturbance in the humid tropics

Abstract

By the peak of land-use conversion for subsistence cropping and plantation agriculture in Puerto Rico in the 1940's, 94 percent of the original forest cover had been eliminated. In a small (26.4 km 2) upland watershed that typifies this land-use history, field surveys and examination of aerial photographs indicate that more than 2,000 landslides have occurred since about 1820 when forest clearing began. The landslides are attributable to a combination of three factors: a highly weathered bedrock (Cretaceous granodiorite), episodic heavy rainfall, and almost two centuries of intense land-use practices. On average, landslide scars number 140/km 2 in the Cayaguás watershed, equal to 80 landslide scars/km 2 /100 y. The volume of hillslope material eroded by landsliding is estimated at 660,000 m 3 /km 2 (870,000 Mg/km 2). If all colluvium was transported from the catchment, then the volume is equivalent to a mean surface lowering of the entire watershed by 660 mm, or 3.8 mm/y. Soil augering, field observations at construction sites, road cuts and stream banks, mapping from aerial photographs, and GIS-based estimates of the surface area of footslopes, indicate that colluvium may total 149,000 Mg/km 2. If mobilized, this would be sufficient stored material to supply the annual average fluvial sediment yield for as long as 129 years. The great availability of colluvial and alluvial sediment on footslopes, floodplains, and in channels will maintain high sediment yield well into the 21st century in spite of government efforts to reforest hillslopes and institute other hillslope soil conservation measures. Wieczorek, G.F., Preparing a detailed landslide-inventory map for hazard evaluation and reduction, Bulletin of the Association of Engineering Geologists, 21, 337-342, 1984.

Potential Effects of Runoff, Fluvial Sediment, and Nutrient Discharges on the Coral Reefs of Puerto Rico

“…In 1939, active cropland reached a high of 48% (BIRDSEY and WEAVER, 1987). Active cropland is a major source of sediment that is entrained during storms and transported to coastal areas (LARSEN and SANTIAGO-ROMÁ N, 2001). Agricultural land in Puerto Rico has been observed by the authors to be in frequent transition between active cropland and pasture, which likely maintains high sediment yield in these areas (GELLIS et al, 2006).…”

“…CRUISE and MILLER (1994) Calvesbert, 1970) and (B) cumulative precipitation associated with Hurricane Georges, September 21-22, 1998 (modified from U.S. Geological Survey, 1999). (GELLIS et al, 2006;GUPTA, 2000;LARSEN and SANTIAGO-ROMÁ N, 2001;MEADE et al, 1990;SOMMERFIELD and NIT-TROUER, 1999). MEADE et al (1990) demonstrated that sediment is discharged during 1% (about 4 days) of the year in North American rivers and varies regionally.…”

“…Extensive hillslope erosion associated with 19th and 20th century agricultural practices in the upper Río Grande de Loíza watershed is estimated to have delivered annually more than 5000 t km Ϫ2 of hillslope-derived sediment to river channels for decades (LARSEN and SANTIAGO-ROMÁ N, 2001). The erosion of hillslope soil cover was estimated to be equivalent to lowering the entire watershed by 66 cm (BROWN et al, 1998).…”

“…These landslides and debris flows produce large volumes of sediment that are available for transport by streams and rivers (LARSEN and SANTIAGO-ROMÁ N, 2001). …”

Planning for an uncertain future - Monitoring, integration, and adaptation

“…Two watersheds (Cayaguás and Canóvanas) have been affected by agricultural activities typical of eastern Puerto Rico (pasture, coffee, tobacco, fruit crops), which led to the loss of much of the original surface layer of soil in these watersheds. Larsen and Santiago Román (2001) estimate that erosion in the Cayaguás watershed, which was intensely farmed for two centuries, lowered the mean surface elevation in the watershed by 660 mm and introduced massive amounts of sediment to river channels, where much of it was deposited as alluvium. This sediment continues to be remobilized during large storm events.…”